In microwave radio link digital transmission there is the need of increasing, as far as possible, the information transmitted per unit of occupied band.
The microwave transmission is typically carried out by transmitting two distinct signals on two right-angled "polarizations" of the electromagnetic field, hereinafter indicated by H and V.
Because of the non-ideal nature of the hardware and transmissive medium, signals H and V tend to interfere with each other considerably.
There are two conventional ways for making negligible the effects of such interference.
The first way consists in transmitting signals H and V on bands which are only partially overlapped, as indicated in FIG. 1.
In this way the portion of (e.g.) signal H which really interferes with V is small and the interference is tolerable. The interference zones are comprised between points I1-I2, I3-I4, etc.
The more efficient second way consists in the so-called re-use of frequency. In this instance the channeling of signals H and V may be represented as in FIG. 2, from which it appears that the transmissive capacity is doubled with respect to FIG. 1, but the two overlapped spectra (e.g. H1 and V1, H2 and V2, etc.) interfere completely.
In this instance the interference of (e.g.) H on V is much more sensible and is opposed through a suitable cancellation circuit which allows the cancellation of the interference itself.
The operation principle of the cancellation circuit, e.g., cancellation of the interference of H on V, is well known: the receiver of the V signal has also the received H signal; the cancellation circuit processes the H signal in such a way as to obtain a replica Hr which is as true as possible to said interference of H present on V.
The cancellation effect is obtained by subtracting the replica Hr (processed by the cancellation circuit) from signal V.
The usual hardware implementation of this system contemplates that receiver of V signal has at its disposal the received H signal downstream of the demodulator of H itself.
What has been described up to now is also true if V is replaced by H and H is replaced by V.
One of the drawbacks arising from the use of such a system is that when one of the two demodulators does not operate correctly (because of failure, adverse propagation or any other reason), the other receiver does not have the signal necessary for cancellation and, therefore, it is subjected to a sensible, no longer cancelable interference. In other words, if one of the demodulators fails, it not only loses its useful signal (e.g. H) but, worse still, it eliminates the possibility of operation also from the second demodulator (e.g. V). After all, one has the complete loss both of H and V, i.e. of all transmission on that carrier.